Use of Google Assistant (Hey Google) comprises of me having to select the Bluetooth  as a audio source to hear prompts and replies. Which kinda shuts up the radio portion. 

I expected it to use the phone Bluetooth connection cutting the audio and resuming once finished 

Has anyone managed to get it set up so it cuts in as and when it should (like a phone call) while still using the radio

I didn't use hey Google I just held the car voice button down. Note you do need to hold it down until you're finished talking unlike the sync commands where you press and let go.

However the fundamental problem with AA for me was I was unable to answer a call or hang up without physically touching my phone which defeats the purpose and makes it illegal.

Thanks for the reply Gillyallan